FIRST OFF, director Darren Lynn Bousman, the man responsible for the last three installments of the “Saw” franchise, does not have fangs, stigmata or blood seeping from his eyelids. Nope, he wears glasses, a baseball cap, jeans and sneakers. Sometimes, he lets his beard get stubbly. Basically, he’s a film geek.

And unlike the hero of his films, he does not stand around glowering under flickering fluorescent lights as he tries to figure out how best to torture people. He does, however, think a great deal about good ways to inflict pain.

“There’s only a certain amount of ways that you can maim someone or kill someone before it just becomes boring,” Bousman says in one of the bonus features included on the DVD for “Saw IV” (Lions Gates, $29.95), in stores Tuesday.

Just like documentaries about Nazi war crimes, where the banal logistics of a brutal regime intersect with demonic acts, the featurettes on “Saw IV” reveal the everyday decisions that go into creating some of the most horrific images captured on film – the kind that have initially garnered every “Saw” movie an NC-17 rating before being edited down to R.

We get to meet production designer David Hackl, who’s in charge of creating and fabricating increasingly intricate “traps” for the victims of Jigsaw, a mechanical engineer turned serial killer who punishes his targets by exploiting their foibles and past sins.

“We try to imagine what will appeal to fans,” he says. “Quite often it will just be an object. And we say, ‘Good god, imagine what that could do to you!’ “

So this time around, there’s not just the usual iron maiden-like head wraps and chains. Now we get entire rooms with a character stuck in a noose standing on a melting ice block. If he slips off, he’ll trigger the electrocution of the guy sitting in the chair next to him.

There’s also property master Jim Murray, a mild-mannered guy who will casually explain how his team turned a nail gun into a “spike gun” that will ultimately fire a lance through the neck of an unwitting FBI agent. He’s also the guy responsible for coming up with a “spine cutter” that threatens to leave an unlikable lawyer paralyzed if he doesn’t do Jigsaw’s bidding.

Good times.

But these are also the people who delight in making Bousman fund daily “Five-Dollar Draws” that give away money to the crew. And the director goes through great lengths – 30 takes, according to one bystander – to have his fluffy white dog Chance do a cameo as a hotel mutt who licks one of Jigsaw’s freaky pig masks.

After watching ultra-gross films such as “Saw” and “Hostel,” it’s actually reassuring to see that there are somewhat normal people responsible for making them. It somehow reaffirms the notion that everybody else who’s seen these movies is just like you – average, decent people who would never actually hurt anybody but sometimes get a weird kick from seeing fellow humans pretend to suffer in unimaginable ways.

In the end, though, Bousman knows what really matters. Fans can’t get enough of gore linked to the twisted mind of a serial killer. It puts butts in the seats and money – nearly $300 million in US tickets sales for all four installments – in the bank. Especially Bousman’s bank.

“I’m not signing that! Go f – – – yourself!” he yells mostly in jest at one of the producers as he signs his contract on-set. “There – now get the f – – – off my case. Go deposit my money!”